Method and means for testing the calibration of radar systems



Sept. 13, 1960 L. a. ZAHALKA ETAL 2,952,848

METHOD AND MEANS FOR TESTING THE CALIBRATION OF RADAR SYSTEMS OriginalFiled April 21. 1953 2 Sheets-Sheet 1 UNIT TARGET COURSE GENERATOR vPHILIP C. FEFFER RICHARD G. HILL LELAND B. ZAHALKA IN VEN TORS Ev M P1960 L. B. ZAHALKA ETAL 2,952,848

METHOD AND MEANS FOR TESTING THE CALIBRATION OF RADAR SYSTEMS 2Sheets-Sheet 2 Original Filed April 21. 1953 PHIL c. FEFFF/Q E/Cf/AEO G.Mu LEZ/M'D 5. 241 /41/01 zzvmvrozas BY M METHOD AND MEANS FQR TESTlNGTHE (I'ALI- BRATION 015 AR SYSTEMS Leland B. Zahalka, Baltimore, andPhilip C. Fetter, Baltimore County, Md., and Richard G. Hill, deceased,late of Ontario, Califi, by Clifton L. Allen, administrator, Pomona,Calif, assignors to Aircraft Armaments, Inc., Baltimore, Md., acorporation of Ohio Continuation of appiication Ser. No. 350,002, Apr.21, 1953. This application Dec. 24, 1958, Ser. No. 784,416

20 Claims. (Cl. 34317.7)

This invention relates to a method and apparatus for testing thecalibration of pulse radar systems and particularly those of the conicalscan type commonly used in fire control systems.

The principal use of radar is to provide information concerning therelative position and movement of a distant object. This information maybe presented directly on an indicator such as a cathode ray tube or itmay also be employed in a computer such as a lead angle computer toprovide data for directing the guns of a fire control system. In theseuses, it is obviously essential that the radar as well as the othercomponents of the system be accurately calibrated and aligned at alltimes so as to provide reliable data.

An object of this invention is to provide a simple and dependable methodand apparatus for dynamically test ing the calibration of a radarsystem.

Another object of this invention is to provide a target course generatorfor generating an imaginary target moving in space along a selected pathand at selected rates so that the radar system being tested is calledupon to operate as it would in actual practice.

Still another object of this invention is to provide an apparatus whichmay be conveniently packaged so as to be: readily portable.

Further and other objects will become apparent from a reading of thefollowing description taken in connection with. the accompanyingdrawings wherein. like numerals refer to like parts.

In the drawings:

Figure 1 is a schematic view of a typical conical scan radar system inan arrangement illustrating the method of this invention.

Figure 2 isa schematic view of the target course generator of thisinvention.

In order to understand the operation of the target course generator asused in the method, it is necessary to understand certain features ofthe operation of a conical scan radar system. Therefore, such a radarsystem is shown schematically in Figure 1 and briefly described hereinbelow as a part of the specification. Radar antenna 4 is carried by ashaft 5 rotatively mounted in a support 6 and is angularly oflset fromthe axis of shaft 5 as indicated by lines 7 and 7' in Figure 1 whereinline 7 represents the centerline of shaft 5 and 7' represents the energytransmitting centerline of the antenna. Thus, by rotating shaft 5, theRF energy transmitted through antenna 4 is caused to produce a conicalbeam pattern. Rotation of shaft 5 is accomplished by operating spinmotor 8 which is suitably mounted on support 6 and mechanicallyconnected to the shaft through suitable gearing 9'. The instantaneousposition of the beam of electrical energy transmitted through antenna 4is ascertained through the use of a reference generator 10 mounted onsupport 6 so as to be suitably operatively connected to shaft 5 such asby gearing 9' through gear 11. The output from reference generator 10may be a Patented Sept. 13, 1960 signal voltage of sinusoidal wave formwhere one full wave represents 360 degrees of rotation of shaft 5-. Thisreference generator signal is fed into radar unit 3 and compared withthe amplitude and phase of reflected energy from a target to provideinformation for directing antenna 4 so that the target will be centeredwithin the conical beam of RF energy. The variation in amplitude of thereflected energy indicates the magnitude of the antenna pointing errorand the phase, as compared with the reference generator output,represents the direction of the error.

Support 6, as schematically shown in Figure 1., is mounted for movementin two mutually perpendicular planes whereby antenna 4 may be moved inazimuth and elevation for zeroing the pointing error An elevation drivemotor 12 and azimuth drive motor 13 control the azimuth and elevationmovement of the antenna in accordance with outputs :15 from radar unit 3Radar unit 3 as shown in Figure 1 may include a computer to operate onrange and rate information concerning the target to introducecorrections which are necessary for a fire control system. The resultingradar information, whether operated on by a computer or not, is employedto present graphical information concerning target position and movementon an indicator 16 such as a cathode ray tube Target course generator 1as shown in Figure 2 includes an antenna 2 arranged to receive pulsed RFenergy from the radar system The received RF energy is fed into suitableRF plumbing 17 and divided so that a major portion of the pulsed energyis directed down one leg 30 through a conventional power attenuator 19,through a hybrid waveguide junction 2t commonly known as a magic-T andfinally to a crystal detector 18 The pulsed RF energy flowing in leg 31is fed. into a conventional transmit-receive (T-R) tube 21 anddissipated therein A klystron local oscillator 22 is provided in thetarget course generator circuit which generates a continuous wave (C.W.)signal at a frequency equal to the RF carrier frequency of the receivedpulse energy from the radar system plus 50 megacycles. The output 23 oflocal oscillator 22. is combined with the pulsed energy transmitted bythe radar system in magic-T 2t and fed to detector 18. The oscillatoroutput is beat with the RF pulsed energy of 5-0 megacycles having thesame pulse width and repetition rate as the receiving RF energy Anattenuator 51. is interposed between local oscillator 22 and magic-T 2Gfor controlling the power of the local oscillator output. The 50megacycle pulsed energy from detector 18 is amplified in. IF amplifier 24- and fed to a; second detector '25 and also to a conventionalautomatic frequency control. circuit 26. The output from automaticfrequency control circuit 26 is fed into local oscillator 22 to. providea stabilizing influence maintaining the frequency of the localoscillator output within close limits.

Detector 25 is responsive to the 50 megacycle pulses from amplifier 24to produce a short output pulse at the start of each. 50 megacyclepulse. The short pulses from detector 25 are fed into an amplifyingblocking oscillator 27 to produce constant amplitude pulses ofsuflicient magnitude to trigger variable delay multivibrator 28.

Triggering multivibrator 28 with the output from blocking oscillator 27starts a generally square wave pulse, the duration of which depends uponthe setting of the multivibrator. The pulse from multivibrator 28 isdifferentiated in differentiator 29 and the resulting signal is fed to ablocking oscillator 32 where it is inverted and clipped to produce asharp control pulse which appears pulse. Thus by varying the time delayof the multivibrator, through rotation of shaft 35, the control pulsesfrom blocking oscillator 32 may be caused to appear at any desired timeafter the start of the pulses of electrical energy transmitted by theradar system.

Output 33 from reference generator 10 of the radar system is fed totarget course generator 1 as shown in Figure 1 and received by avariable phase shifter 34 as shown in Figure 2. The phase of thereference generator signal is shifted as desired from degrees to 360degrees by the rotation of shaft 36. The output from phase shifter 34 isfed through lead 37 to a smoothing device 38 known as a thermistor tomaintain the phase shifted output at a desired and uniformly varyingamplitude. Output 39 from the smoothing device 38 is fed to an amplitudemodulator 40 for amplitude modulating a 50 megacycle continuous Wave(C.W.) signal generated by a crystal controlled oscillator 41. Referencegenerator 10 may obviously become a part of the target course generatorrather than a part of the radar system. The only requirement is that thereference generator signal be synchronized with the scanner rotation. r

The amplitude modulated 50 megacycle C.W. signal output from modulator40 is fed to a gated amplifier 42 through lead 43. The control pulseoutput from blocking oscillator 32 is fed to gated amplifier 42 throughlead 44 for controlling the gate whereby the amplitude modulated 50megacycle signal is allowed to pass the amplifier only in response to acontrol pulse from the blocking oscillator so as to produce pulses, theduration of which are substantially equal to the duration of the pulsestransmitted by the radar system. Since the control pulses are generatedfrom the pulses transmitted by the radar, the repetition rate of theamplitude modulated 50 megacycle pulses from gated amplifier 42 willalso be the same as the repetition rate of the radar transmitted pulses.

The pulsed output from gated amplifier 42 is fed to a crystal detector45 through lead 46. The SO'megacycle pulsed output from detector 45 iscombined with the output from local oscillator 22 in a magic-T 47. Sincethe local oscillator output is at a frequency equal to the RF frequencyof the received pulse energy from the radar system plus 50 megacycles,when it is combined with the 50 megacycle pulsed output from detector 45a pulsed output results which is at two frequencies, one being at the RFfrequency of the received pulse energy from the radar system, the otherbeing at the received frequency plus 100 megacycles. This output is fedthrough a power attenuator 48, through T-R tube 21, which allows energyto pass only in the direction of the arrow 52, and on to antenna 2 wherethe energy is directed towards radar antenna 4 so that it may be pickedup as a simulated echo pulse from an imaginary target. The frequency ofthe echo pulse which is at the RF frequency plus 100 megacycles issufiiciently outside the pass band frequency of the radar so that onlythe RF frequency of the echo pulse is picked up.

Shaft 35 controlling variable delay multivibrator 28, and shaft 36controlling variable phase shifter 34, are both connected to asynchronous motor 49 to vary the time delay of the control pulse and thephase of the signal from the reference generator continuously and at thedesired rate so as to produce the eifect of a target moving in range,azimuth and elevation along a prescribed path which may be accuratelyreproduced by the target course generator at any time.

The dynamic aspects of the tests performed by the disclosed system, andan explanation of how the pulses retransmitted to the radar setcorrespond to the signals reflected from a moving target, will now begiven. As pointed out above, the principle upon which the invention isbased, is a recognition that in a conical scan system utilizing arotating antenna, a signal produced by the re flected energy from atarget when compared to a signal from a reference generator synchronizedwith said antenna, will diifer in amplitude and phase whenever thetarget is not centered within the conical beam of RF energy. Thevariation in amplitude is a measure of the magnitude of the pointingerror, and the shift in phase is a measure of the direction thereof.Normally, this principle is utilized to zero the pointing error and sotrack a moving target. Range of the target, in such case, is in dicatedby the time delay as between the reflected RF energy and the outputenergy.

If the pointing error were not zeroed, a moving target would produce areflected signal, which when compared to the antenna output would bedelayed in time, and which when compared with the signal from thereference generator would vary in amplitude and be in phase shiftedrelation thereto. Thus, a moving target can be simulated by using someof the radar antenna output to produce a signal having the abovedescribed characteristics and transmitting the same to the radar set.Such a signal is produced by variable delay 28, amplitude modulator 40,and variable phase shifter 34, respectively.

Variable delay multivibrator 28 is of a conventional type whichproduces, in response to a trigger pulse, an essentially square pulsewhose leading edge coincides with the trigger pulse, and the length ofwhich is variable according to the setting of the multivibrator. Thetrailing edge of this square pulse is used to produce a trigger pulsefor gated amplifier 42. Since the output of the amplifier 42 is a seriesof pulses having the same repetition rate as the output of the radarantenna, the delay of the pulses with respect to the antenna output maybe varied by varying the setting of the multivibrator, therebysimulating pulses from a target changing in range.

The amplitude modulator 40. modulates the output of the crystaloscillator 41 in accordance with the output from the reference generator10, so that the pulse output of gated amplifier 42 is modulated tosimulate the reflected signal from a target displaced from the axis ofrotation of the radar antenna 4. When the amplitude of the modulation isconstant, the signal received by antenna 4 appears to be that from atarget moving in azimuth at a constant rate. V g g Variable phaseshifter 34 shifts the phase of the modulating signal applied tomodulator 40, thereby shifting the phase of the output of gatedamplifier 42 with respect to the output of thereference generator 10.Since the amount of this phase shift is related to the angular positionof the target in a plane normal to the axis of rotation of antenna 4, acontinuous shiftin phase of the output of amplifier 42 simulates thereflected signal from a target which is changing its angular position insuch a plane.

Programming of a target may be accomplished, therefore, by changing theamount of modulation in modulator 40 to change the pointing error; byincreasing or decreasing the length of the square wave produced bymultivibrator .28 to cause the target to appear to move toward or awayfrom the antenna 4; or by varying the phase shift introduced by phaseshifter 40 to vary the apparent angular position of the target aroundthe axis of rotation of the antenna. Any particular path may be selectedby properly relating the variations introduced, the particularconfiguration shown in Figure 2 of the drawing being intended tocontinuously lengthen and shorten the pulse output of multivibrator 28to cause the target to appear to approach vand then recederfrom theantenna, while the phase of the output of amplifier 42 is continuouslyshifted to cause the target to appear to rotate around the axis ofrotation of antenna 4, thus producing a signal output at antenna 2 whichsimulates the reflection from a target which approaches and recedeswhile traveling a spiral path. Any other preselected path may beprogrammed by suitably varying the amplitude of the modulation effectedby modulator 40, the amount of delay introduced by the multivibrator 28,and the rate of phase shift in the variable phase shifter 34;

In the practice of the method, the following Procedure sented on theindicator.

is followed. At a time when the radar system is properly adjusted,target course generator 1 is located .relative to the radar system sothat its antenna 2 is supported closely adjacent radar antenna 4 forreceiving a portion of the pulsed energy transmitted by the radar systemwhen operating. A camera 50 or other suitable recording device isarranged relative to the target position indicator 16 for making apermanent recording of the data pre- The radar system is then turned onso as to operate in the normal manner, transmitting RF energy. A portionof the RF energy is picked up by antenna 2 of the target coursegenerator and employed therein as previously described to produce apulsed output that is radiated from antenna 2 and picked up by radarantenna 4 as simulated echo pulses. The path of the target as indicatedon indicator 16 is recorded by camera 50. Then at a subsequent time,when it is desired to test the calibration of the radar system, thetarget course generator is again arranged adjacent radar antenna 4 asherinbefore described and is caused to generate an imaginary targetmoving in range azimuth and elevation along the same path as before bycontrolling the rotation of shafts 35 and 36 in the same manner throughthe operation of motor 49. The path of the moving target as presented onindicator 16 is compared with the prior path recorded by the camera.This comparison may be made by making a transparent or translucentoverlay from the original photographic negative and placing it over theface of the radar indicator or the indicator may again be photographedand the negative thus made compared with the negative made when theradar system was known to be properly adjusted. If the paths aspresented by the indicator are similar, the radar system is properlycalibrated and adjusted. If the indicated paths are different, the radarsystem is not functioning properly and maintenance is required so thatthe recorded path will again be duplicated.

The method hereinbefore described provides a dynamic test for checkingthe calibration of radar systems whereby the operation of all componentsof the system are investigated while functioning under the sameconditions encountered in actual practice.

Though oscillators 22 and 41 in the target course generator aredescribed as having specific output frequencies, it is merely by way ofexample and is not intended to limit the invention since any frequencyin the general range of those specified will be equally satisfactory solong as the frequency of the output signal from oscillator 41 issubstantially the same as the frequency difference between the outputfrom local oscillator 22 and the received RF signal.

It is to be understood that certain modifications, substitutions andalterations may be made without departing from the spirit and scope ofthe invention as defined by the appended claims.

This application is a continuation of application Serial No. 350,002,filed April 21, 1953 (now abandoned).

What is claimed is:

l. A test set of the type used to calibrate range in a radar setcomprising: means to receive a radar pulse from a radar set, meansincluding a first oscillator to convert said radar pulse to an IF pulse,means to convert said IF pulse to a video pulse, means to convert saidvideo pulse to a trigger pulse delayed in time with respect to saidradar pulse, means including a second oscillator responsive to saiddelayed trigger pulse to produce a pulse having the same delay as saiddelayed trigger pulse and substantially the same frequency as said IFpulse, means including said first oscillator to convert said delayed IFpulse to an artificial echo radar pulse delayed in time with respect tosaid radar pulse and substantially the same frequency as said radarpulse, and means to transmit said artificial echo radar pulse.

2. In combination, means to receive an RF input pulse,

means including a first oscillator to convert said input pulse to an IFinput pulse, means to convert said IF input pulse to a video pulse,means to convert said video pulse to a trigger pulse delayed in timewith respect to said RF input pulse, means including a second oscillatorand responsive to said delayed trigger pulse for producing an IF outputpulse having substantially the same delay as said delayed trigger pulseand the same frequency as said IF input pulse, and means to convert saiddelayed IF output pulse to an RF output pulse delayed in time withrespect to said RF input pulse and substantially the same frequency.

3. .A radar test set comprising first means to convert an RF radar pulseto an IF pulse, means to convert said IF pulse to a video pulse, meansresponsive to said video pulse for producing a trigger pulse delayed intime with respect to said RF radar pulse, a continuous wave oscillatorhaving an output of the same frequency as that of said IF pulse, meansresponsive to said delayed trigger pulse to convert said output to adelayed IF pulse, and second means including a part of said first meansto convert said delayed IF pulse to an RF echo pulse of substantiallythe same frequency as said RF radar pulse.

4. A target course generator for testing the calibration of an objecttracking radar system having directional antenna means for transmittingand receiving pulsed electrical energy and means responsive to the timedelay between transmitted and received pulses and to the phase andamplitude of the envelope of said received pulses to determine targetrange, azimuth and elevation movement comprising: an oscillatorgenerating a continuous wave signal, means including variable phaseshifting means generating a modulating signal at a phase depending uponsaid variable phase shifting means, modulating means responsive to saidmodulating signal for modulating said continuous wave signal, controlpulse generating means including variable time delay means responsive topulses transmitted by said radar system for producing control pulsesdelayed in time from the radar pulses by an amount depending upon saidvariable time delay means, gating means connecting with the output fromsaid modulating means and responsive to said control pulses forintermittently passing the modulated continuous wave signal to producesimulated echo pulses and means for transmitting said simulated echopulses to said radar system to produce an imaginary target moving inrange according to the control of said time delay means and in azimuthand elevation according to the control of said variable phase shiftingmeans.

5. A target course generator for testing the calibration of a conicalscan radar system having a directional antenna mounted for rotationalmovement so as to direct a cone shaped beam of pulsed electrical energyoutwardly therefrom and a reference generator responsive to therotational movement of said antenna to produce an output signal voltagevarying in accordance with the antenna movement whereby to provide meansfor determining the instantaneous position of said beam comprising:receiving means for receiving said pulsed electrical energy, controlpulse generating means including variable delay means electricallyconnected to said receiving means and responsive to said pulsedelectrical energy for generating control pulses, said control pulsesbeing delayed in time relative to said electrical energy pulses by anamount depending upon the setting of the variable delay means, anoscillator for generating a continuous wave signal, a variable phaseshifter responsive to the output signal from the reference generator andhaving an output signal similar thereto in wave form and out of phasewith said reference generator output by an amount depending upon thesetting of said phase shifter, modulating means for amplitude modulatingthe output signal from said oscillator With the output signal from saidphase shifter to provide an amplitude modulated continuous wave signal,the envelope of which is shifted out of phase with the referencegenerator signal by the operation of said phase shifter, transmittingmeans for transmitting said amplitude modulated signal so as to bereceived by said radar system, gating means interposed between saidtransmitting means and said modulating means and responsive to saidcontrol pulses for controlling the passage of said amplitude modulatedsignal to produce pulses at substantially the same repetition rate assaid pulsed electrical energy transmitted by said radar system toprovide simulated echo pulses representing reflections from an imaginaryobject, the position of which relative to said radar system antennadepending upon the control of said phase shifter and said delay means.

6. A target course generator for testing the calibration of an objecttracking radar system having directional antenna means for transmittingand receiving pulsed electrical energy and means responsive to the pulsetime, phase and amplitude of the received energy for determining thelocation and movement of an object comprising: an oscillator forgenerating a constant amplitude continuous wave signal, a signalgenerator including variable phase shifting means generating a secondcontinuous wave signal at a phase depending upon the setting of saidphase shifting means, modulating means responsive to said oscillator andto said signal generator for amplitude modulating the oscillator outputwith said second continuous wave signal, receiving means for receivingsaid pulsed electrical energy from the radar system, control pulsegenerating means including variable time delay means electricallyconnected to said receiving means and responsive to said pulsedelectrical energy for generating control pulses delayed in time from thepulses in said pulsed electrical energy by an amount depending upon thesetting of said variable time delay means, gating means connecting withthe output from said modulating means and responsive to said controlpulses to pass the output from said modulating means as pulses startingwith each control pulse, and transmitting means responsive to the pulsedoutput from said gating means for transmitting the pulsed output to saidradar system as simulated echo pulses from an imaginary target moving inrange according to the control of said time delay means and in azimuthand elevation according to the control of said variable phase shiftingmeans.

7. A target course generator for testing the calibration of an objecttracking radar system having directional antenna means for transmittingand receiving pulsed electrical energy and means responsive to the timedelay between transmitted and received pulses and to the phase andamplitude of the envelope of the received pulses to determine targetlocation and movement comprising: receiving means for receiving saidpulsed electrical energy transmitted by said radar system, oscillatingmeans generating an output signal at a frequency different from thefrequency of the radar pulse carrier frequency, said receiving meansbeing responsive to said oscillating means output to produce pulses at acarrier frequency equal to the frequency difference between theoscillating means output frequency and the radar pulse carrierfrequency, control pulse generating means including variable time delaymeans responsive to said receiving means for producing control pulsesdelayed in time from the radar transmitted pulses by an amount dependingupon the setting of said variable time delay means, oscillating meansgenerating a continuous wave signal at a frequency substantially equalto the frequency difference between the radar pulse carrier frequencyand the output frequency of the first mentioned oscillating means, asignal generator generating a reference signal, variable phase shiftingmeans responsive to said signal generator for producing a secondcontinuous wave signal at a phase relative to said reference signaldepending upon the setting of said phase shifting means, modulatingmeans responsive to the last mentioned oscillating means and to theoutput from said phase shifting means for amplitude modulating theoutput from said last mentioned oscillating means with said secondcontinuous wave signal, gating means connecting with the output fromsaid modulating means and responsive to said control pulses for passingthe output from said modulating means intermittently as pulses beginningin point of time with each control pulse, and transmitting meansresponsive to the pulsed output from said gating means and from theoutput of said first mentioned oscillating means for transmitting tosaid radar system a pulsed output at the frequency of the radar pulsecarrier frequency representing simulated echo pulses from an imaginarytarget moving in range according to the control of said time delay meansand in azimuth and elevation according to the control of said variablephase shifting means.

8. A target course generator for testing the calibration of an objecttracking radar system having directional antenna means for transmittingand receiving pulsed electrical energy and means repsonsive to the phaseand amplitude variation of the received energy for determining thelocation of an object comprising: receiving means for receiving pulsedelectrical energy transmitted by the radar system, control pulsegenerating means electrically connected to said receiving means andresponsive to said pulsed energy transmitted by the radar system forgenerating control pulses, an oscillator generating a constant amplitudecontinuous wave signal, signal generating means including a variablephase shifter generating a second continuous wave signal, the phase ofwhich relative to a reference phase depends upon the setting of saidvariable phase shifter, modulating means responsive to the oscillatoroutput and to the output from said phase shifter for amplitudemodulating the oscillator output, time variable gating meanselectrically connecting with the output from said modulating means andresponsive to said control pulses to intermittently pass the output fromsaid modulating means whereby to produce an electrical pulse train, theenvelope of which varies in amplitude and phase in accordance with saidsecond continuous wave signal to provide simulated echo pulses of animaginary target moving in space, and transmitting means responsive tothe output from said gating means for transmitting said electrical pulsetrain so as to be received by said radar system whereby said system iscaused to respond according to the phase and amplitude of the receivedenergy from said course generator and indicate a moving target.

9. A target course generator for testing the calibration of a conicalscan pulse radar system having a directional antenna mounted forrotational movement about air axis offset from the antenna axis and areference generator producing a voltage proportional to theinstantaneous rotational position of said antenna relative to a zeroreference position comprising: an antenna arranged to receive electricalpulses transmitted by said radar system, variable pulse delay meanselectrically connected to said last mentioned antenna and responsive tosaid transmitted pulses for generating control pulses delayed in timefrom said transmitted pulses by an amount depending upon the setting ofsaid pulse delay means, an oscillator generating a continuous Wavesignal, means including a variable phase shifter responsive to theoutput from said reference generator and having an output phase shiftedwith respect to said reference generator output by an amount dependingupon the setting of said phase shifter, modulator means responsive tothe oscillator output and the output from said phase shifter foramplitude modulation of said oscillator output, gating meanselectrically connecting with the modulator output and responsive to saidcontrol pulses for passing the modulator output intermittently assimulated echo pulses at a repetition rate equal to the control pulserepetition rate, and transmitting means responsive to the output fromsaid gatingmeans for transmitting said simulated echo pulses to saidradar system to produce an indication of an imaginary target 9 theposition of which relative to said radar system being determined by thecontrol of said variable pulse delay means and the control of saidvariable phase shifter.

, 10. A target course generator for testing the calibration of an objecttracting radar system having directional antenna means for transmittingand receiving pulsed electrical energy and means responsive to the phaseand amplitude variation of the received energy for determinting thelocation of an object comprising: receiving means for receiving pulsedelectrical energy transmitted by the radar system, an oscillatorgenerating a constant amplitude continuous wave signal, signalgenerating means including a variable phase shifter generating a secondcontinuous Wave signal, the phase of which relative to a reference phasedepends upon the setting of said variable phase shifter, modulatingmeans responsive to the oscillator output and to the output from saidphase shifter for amplitude modulating the oscillator output, timevariable gating means electrically connecting with the output from saidmodulating means and responsive to said receiving means tointermittently pass the output from said modulating means whereby toproduce an electrical pulse train, the envelope of which varies inamplitude and phase in accordance with said second continuous wavesignal to provide simulated echo pulses of an imaginary target moving inspace, and transmitting means responsive to the output from said gatingmeans for transmit-ting said electrical pulse train so as to be receivedby said radar system whereby said system is caused to respond accordingto the phase and amplitude of the received energy from said coursegenerator and indicate a moving target.

11. The method of testing the calibration of an object tracking radarsystem having a graphical position indicator and energy transmitting andreceiving means for determining target range, azimuth and elevationmovement comprising: generating energy simulating an echo signal from atarget moving in range, azimuth and elevation along a selected path,transmitting said energy so as to be received by said radar system forproducing an indication of target movement on said graphical indicator,placing a translucent overlay having the known correct presentationmarked thereon on said graphical indicator, and comparing the knowncorrect presentation with the presentation on the radar indicator.

'12. The method of testing the calibration of an object tracting radarsystem having a graphical indicator, energy transmitting and receivingmeans and means responsive to the energy transmitting and receivingmeans for presenting target movement on said graphical indicatorcomprising: generating energy simulating an echo signal from a targetmoving in range, azimuth and elevation along a selected path,transmitting said energy so as to be received by said radar system forproducing an indication of target movement on said graphical indicatorwhen said radar system is properly calibrated, photographing saidgraphical indicator to provide a record of the presentation on aphotographic negative, making an overlay from said photographicnegative, then generating, when the calibration of said radar system isto be checked, energy simulating an echo signal from a target moving inrange, azimuth and elevation along the previously selected path,transmitting said energy so as to be received by said radar system forproducing an indication of the target movement on said graphicalindicator and placing said reverse negative overlay on said graphicalindicator and comparing the path indicated thereon with the pathrecorded on said reverse negative.

13. The method of testing the calibration of a radar system having agraphical position indicator and energy transmitting and receiving meansfor determining target range, azimuth and elevation movement comprising:transmitting a radar signal, utilizing said signal to produce an echosignal simulating a target moving in range, azimuth and elevation alonga preselected path, transmitting said echo signal so as to be receivedby the receiving means of said radar system for producing an indicationof target movement :on said graphical indicator, placing on saidgraphical indicator an overlay having the known correct presentationmarked thereon, and comparing the known correct presentation with thepresentation on the radar indicator.

14. The method of testing the calibration of a. radar system having agraphical position indicator and energy transmitting and receiving meansfor determining the range of a moving target comprising: transmittingradar energy, utilizing said energy to produce an echo signal simulatinga target moving in range along a preselected path, retransmitting saidecho signal so as to be received by the receiving means of said radarsystem for producing an indication of target movement on said graphicalindicator, placing on said graphical indicator an overlay having theknown correct presentation marked thereon, and comparing the knowncorrect presentation with the presentation on the radar indicator.

15. A radar test set comprising: means to receive a radar pulse from aradar set, means including a first oscillator to convert said radarpulse to an IF pulse, means to automatically tune said first oscillatorto a frequency removed from the frequency of the radar pulse by anamount equal to the frequency of the IF pulse, means to detect theenvelope of said IF pulse for producing a video pulse, means to convertsaid video pulse to a con- 'trol pulse delayed in time with respect tosaid radar pulse, a'second oscillator tuned to substantially thefrequency of said IF pulse, means for combining the output of bothoscillators for producing a signal at substantially the same frequencyas said radar pulse, said control pulse causing said signal to be apulse delayed'in time with respect to said radar pulse by an amountequal to the delay of said control pulse, and means to transmit saidsignal to said radar set. V I

16. A radar test set comprising: means to receive a radar pulse from aradar set, a first oscillator tuned to a frequency different fromthat ofsaid radar pulse, means to combine the output of said first oscillatorwith said radar pulse for producing an IF pulse, means to detect theenvelope of said IF pulse for producing a video pulse, means to convertsaid video pulse to a control pulse delayed in time with respect to saidradar pulse, means including a second oscillator tuned to a givenfrequency, means for combining the outputs of both oscillators forproducing a signal at substantially the same frequency as said radarpulse, said control pulse causing said signal to be a pulse delayed intime with respect to said radar pulse by an amount equal to the delay ofsaid control pulse, and means to transmit said signal to said radar set.

17. A radar set comprising: means to receive radar pulses from a radarset, a tunable local oscillator for producing an output at a frequencydifferent from that of said radar pulses, means to combine the output ofsaid local oscillator with said radar pulses for producing IF pulses ata frequency equal to the difference between the oscillator frequency andthe radar pulses frequency, AFC means responsive to the frequency of theIF pulses for tuning said local oscillator such that the pulse-to-pulsefrequency of the IF pulses is uniform means to detect the envelope ofsaid IF pulses for producing video pulses in time coincidence with theradar pulses, means to convert said video pulses to control pulsesdelayed in time with respect to said radar pulses, signal generatingmeans including a circuit tuned to substantially the frequency of the IFpulses, said signal generating means being responsive to said controlpulses for producing simulated pulses at substantially the samefrequency as said IF pulses but delayed in time with respect thereto,means to combine said simulated pulses with the output of said localoscillator for producing simulated radar pulses at substantially thesame frequency as said radar pulses but delayed in time with respectthereto, and means to transmit said simulated radar pulses to said radarset.

18. The method of testing the calibration of a radar system having (1)transmitting means for directing energy toward a moving target, (2)receiving means for determining the range, azimuth and elevation of atarget from the transmitted energy that is reflected from the object,and (3) an indicator connected with said receiving means for displayingthe range, azimuth and elevation as indicia representative of the pathof the target comprising the steps of: (1) generating electrical energyaccording to a preselected program for simulating an echo signal from anobject changing range, azimuth and elevation in a manner determined bythe preselected program; (2) applying said electrical energy to saidreceiving means and indicator so that the latter displays indiciarepresentative of a target changing range, azimuth and elevation; and(3) ascertaining the calibration of the radar system by observing therelationship between the displayed range, azimuth and elevation and therange, azimuth and elevation determined by the preselected program.

19. The method of testing the calibration of a radar system having (1)transmitting means for directing energy toward a moving target, (2.)receiving means for determining the range, azimuth and elevation of atarget from the transmitted energy that is reflected from the object and(3) an indicator connected with said receiving means for displaying therange, azimuth and elevation as indicia representative of the path ofthe target comprising the steps of: (1) generating electrical energysimilar to the energy which would be reflected from an object changingrange, azimuth and elevation in a pre selected manner as it moves in apreselected path, (2) applying said electrical energy to saidreceiving-means and indicator when the radar system is properlycalibrated so that the indicator displays calibration indiciarepresentative of the preselective path, (3) making a record of saidcalibration indicia, (4) generating again the energy as in step (1) andapplying the same to said receiving means and indicator when thecalibration of the radar system is to be checked so that the indicatordisplays indicia representative of the path of a target, and (5)ascertaining the calibration of the radar system by observing therelationship between the displayed indicia representative of the path ofthe target and the recorded calibration indicia. I I

20. The method of testing the calibration of a radar system having (1)transmitting means for directing energy toward a moving target, (2)receiving means for determining the range, azimuth and elevation of atarget from the transmitted energy that is reflected from the object and(3) an indicator connected with said receiving means for displaying therange, azimuth and elevation as indicia representative of the path ofthe target comprising the steps of: (1) transmitting energy from saidsystem, (2) utilizing said energy to produce echo energy according to apreselected program for simulating an echo signal from an objectchanging range, azimuth and elevation in a manner determined by thepreselected program, (3) applying said echo energy to said receivingmeans and indicator so that the latter displays indicia representativeof a target changing range, azimuth and elevation, and

(4) ascertaining the calibration of the radar system by observing therelationship between the displayed range, azimuth and elevation and therange, azimuth and elevation determined by the preselected program.

References Cited in the file of this patent UNITED STATES PATENTS2,440,253 Dodington Apr. 27, 1948 2,477,485 Jacob July 26, 19492,477,585 Dodington Aug. 2, 1949 2,489,251 Anast Nov. 29, 1949 2,522,541Saxton et al. Sept. 19, 1950 2,706,773 Dodington Apr. 19, 1955 2,708,148Clark May 10, 1955 2,757,285 Forbes July 31, 1956 FOREIGN PATENTS593,539 Great Britain Oct. 20, 1947 UNITED STATES PATENT OFFICECERTIFICATE F CORECTIUN Patent No, 2 952 848 September 13; 1960 LelandB9 Zahalka et al9 It is hereby certified that error appears in theprinted specification of the above numbered patent requiring correctionand that the said Letters .Patent should read as corrected below.

Column 10, line 62 after "uniform" insert a comma;\

Signed and sealed this 11th day of April 1961.

(SEAL) Attest:

s "ifii iiasT WJ SiK/ IDER ARTHUR W. CROCKER Attesting Oificer A tingCommissioner of Patents UNITED STATES PATENT OFFICE CERTIFICATE OFCORRECTION -Patent No. 2352,8148 September l3 1960 I Leland B. Zahalkaet al. It is hereby certified that error appears in the printedSpecification of the above numbered patent requiring correction and thatthe said Letters .Patent should read as corrected below.

after "uniform insert a comma.

Column 10, line 62 Signed and sealed this 11th day of April 1961.

(SEAL) Attest:

REESE WFsnfibER ARTHUR w. CROCKER Agi g I Commissioner of PatentsAttesting Oificer

